1. Field of the Invention
The present invention relates to a sheet feeder that detects multi-feed of conveyed sheets, and an image forming apparatus.
2. Description of the Related Art
A conventional multi-feed detection device is known which is provided with an ultrasonic transmitter and an ultrasonic receiver, for transmitting ultrasonic waves from the ultrasonic transmitter to a plurality of spots on a sheet material being conveyed, and detecting multi-feed of sheet materials based on an amount of attenuation of the ultrasonic waves received by the ultrasonic receiver (see Japanese Patent No. 3890766).
The above-mentioned conventional multi-feed detection device irradiates a plurality of spots on a sheet material with ultrasonic waves, and determines an occurrence of multi-feed of sheet materials if there are a predetermined or larger number of spots where the amount of attenuation of the ultrasonic waves is larger than a reference value due to superposition of sheet materials.
Variation in distance between the ultrasonic transmitter and the ultrasonic receiver (distance between the sensors) is considered to be a factor degrading the accuracy of multi-feed detection. The variation in distance between the sensors sometimes increases or decreases the level of output from the ultrasonic receiver. The above-mentioned conventional multi-feed detection device cannot cope with this variation in distance between the sensors. Particularly, in a case where the ultrasonic transmitter and the ultrasonic receiver are disposed in a manner spaced from each other so as to make it easier for a user to remove a jammed sheet caused by paper jam occurring in the vicinity of the sensors, the distance between the sensors may be changed whenever a paper jam occurs. Further, when the distance between the sensors has changed, there is a possibility that the multi-feed detection device cannot detect multi-feed of sheet materials, or erroneously detects multi-feed of sheet materials which are being fed normally.